Disaster-Causing Mechanism Of Overburden Rock Failure Due To Mining Under Deep Buried Soil-Rock Interface

A serious water inrush (mud inrush) accident happened to the roof when somemines in the eastern China were mining near the bottom of deep loose layer of soiland the interface of coal weathering rock (referred to as deep buried soil-rockinterface). It often accompanies by pressure abnormal increasing or even supportcrushing. However, the mechanisms of these accidents are difficult to be explainedreasonably by traditional theories. To address this serious coal mine engineeringgeological problems, Anhui Huainan Panxie mine was selected as the research areato make some basic research on the engineering geological properties ofsoil-water-rock interaction under high pressure in deep buried soil-rock interface.And disaster-causing mechanism of overburden failure under the deep buriedsoil-rock interface was analyzed when the coal was mining.Some research methods such as theoretical analysis, field tests, laboratory tests,numerical simulation and practical application were used in this study. Interpretationon overburden failure and disaster-causing mechanism of mining with pressureunder deep buried soil-rock interface was placed first. On the basis of the analysis ofdeep buried soil-rock interface geological environment and mechanical properties ofhigh-pressure direct shear, on the theme of " classification of deep of soil-rockinterfaceâ†'the formation of adown penetration zoneâ†'development law ofwater flowing fractured zoneâ†'the mechanisms of support crushing and waterinrush of working face ", made the following main results:(1) Field geological survey of deep buried soil-rock interface was done, andgeotechnical samples were taken. The various types of logging data and coreidentification methods were used to identify the position of soil and coal weatheringrock interface. Integrated field and laboratory test data, engineering geology type ofdeep buried soil-rock interface can be divided into four types. It reaches a decisionthat the working face where serious roof water inrush accidents occurred is locatedunder deep buried sand-sandstone interface.(2) The direct shearing orthogonal test on characteristics of deep buried sandand coal weathering rock interface was conducted on the DSR-1computerized highstress direct/post shearing system which was self-developed by the State KeyLaboratory for Geomechanics and Deep Underground Engineering, China Universityof Mining and Technology. The variance and range analysis method were used to analyze the sensitivity of each factor on the mechanical properties of the interface.The test results show that normal pressure is a major factor affecting the mechanicalproperties of the interface, and the roughness and lithology of interface aresecondary factors. The curves of shear stress-shear displacement (Ï„-ω) of theinterface as a whole shows a hyperbolic relationship with no obvious peak stress,and shows shear shrinkage characteristics. When the normal pressure is6MPa, theshear strength of the contact surface is consistent in different roughness. When thenormal pressure is larger (≥8MPa), the greater roughness of contact surface is, thegreater shear strength of the contact surface will be. Shear strength and normalpressure of the interface has a good linear relationship, which can be described bythe Mohr-Coulomb criterion.(3) On the basis of survey on waterproof coal pillar set high enough in HuainanPanxie mine and a number of actual water inrush data of fully-mechanized miningface, the concept of adown penetration zone is first put forward. And formation ofadown penetration zone under deep buried sand-sandstone interface should meetthree hydrological and engineering geological conditions. Theoretical formula of thedevelopment depth of adown penetration zone were derived by limit equilibriumtheory and hydraulic fracturing criterion, and the range of development depth ofadown penetration zone in Huainan Panxie mine was given.(4) On the basis of analysis of the geological environment of fractured rockmass with high water pressure under deep buried sand-sandstone interface,development rules of the height of water flowing fractured zone during mining underdeep buried soil-rock interface were discussed according to key strata theory and thetheory of underground pressure control. Based on measured data of the height ofwater flowing fractured zone, key controlling factor on the height of water flowingfractured zone was studied by principal component analysis. According to simplehydrological observations data of drilling hole, the effect of the lithology on theheight of water flowing fractured zone was analyzed and then a clear method waspresented for determining critical plane of water flowing fractured zone. Partition ofdeep buried soil-rock interface was done according to measured data of waterflowing fractured zone. The effect of roof types and mining thickness was studied onthe development of height of water flowing fractured zone.(5) On the basis of characteristic analysis of support crushing and water inrushof working face in Huainan Panxie mine, the existence of adown penetration zone before coal mining and the support crushing and water inrush mechanisms ofoverburden failure and abnormal development after coal mining are proposed. Thenit can be determined that the existence of adown penetration zone is the mostfundamental and most important reason for support crushing and water inrushaccidents. Therefore, the key control techniques for support crushing and waterinrush are that improve support resistance of mechanized mining, accelerate theadvance of mining face, and conduct grouting reconstruction for adown penetrationzone at the same time.